You will first need to leave the beta by clicking on the cogwheel on the upper right hand of the main beta page then tapping on beta status. You will then need to plug your phone into your PC then open up the PC version of smart Switch. Be sure to back up your phone since downgrading requires a factory reset. However, once the official build is released there will be an OTA and you will be able to update without a factory reset
One UI Beta Team.